The invention relates generally to the field of gas lasers which have loss mechanisms and, more particularly, to the field of gas resupply valves for such gas lasers to supply new gas to maintain gas pressure within the laser tube within acceptable limits.
In almost all gas lasers there is a loss mechanism whereby gas molecules trapped in the laser tube are lost either through leaks or through chemical combination with or sputtering into the materials on the inside of the laser tube. In the field of ion lasers, and especially in the field of argon ion lasers, the argon atoms are ionized by electrical discharges passing through the gas-filled laser tube. These ions are then excited to higher energy states by pumping energy supplied from an outside source. Although argon is a noble gas, argon ions can "sputter" into the walls of the laser tube and be lost for further laser action. Because there is a very narrow band of acceptable pressures for gas in the laser tube which will cause lasing action, it is important that the pressure of argon gas in the laser tube remain relatively constant. Thus, when argon ions are lost, pressure in the laser tube will drop. This loss of pressure can be detected by monitoring the voltage appearing across the electrodes which ionize the argon gas since the voltage drops as the pressure drops. If the lost argon ions are not replaced with new gas molecules, the laser will become unstable. If the pressure in the gas tube drops low enough, the laser will stop lasing.
In the prior art, the lost gas has been replaced by the use of gas resupply valves. These valves have small reservoirs for storing argon gas at atmospheric pressure (or any other pressure) and have metering volumes. When the pressure in the laser gets too low, the metering volume is filled with gas through a valve and this gas is then allowed to enter the highly evacuated laser tube to replenish the gas supply. The use of the metering volume allows a known quantity of gas to be injected into the laser tube on each "charging" cycle. Very tiny amounts of gas are involved in this process. This is because if too much gas is allowed to enter the laser tube, the laser can be essentially ruined since higher pressures mean higher voltages between the ionization electrodes which the power supplies are not designed to handle. When too much gas enters the tube, the tube must be sent back to a refurbishing facility for reprocessing to get the pressure back down to an acceptable level.
The requirements for a gas resupply valve for gas lasers are three. First, the valve must have a negligible leak rate when the valve is closed. This is required so that gas laser tubes which sit in inventory unused for many weeks or months do not leak gas to the extent that the laser becomes unusable or inoperative before it is ever used. A second requirement is that the valve have a lifetime of at least 1,000 cycles between open and closed positions without failure or degradation in the residual leak rate when the valve is closed. Finally, such gas resupply valves must have extremely short cycle times between opening and closing in embodiments where only one valve member and a controlled leak aperture is used. There are prior art embodiments to be discussed below which use two valve members. For these valves, it is not necessary that the cycle time be as small as noted above. But all gas resupply valves must meet the first two requirements.
An early gas resupply valve design known to workers in the art used two valve seats and two valve members. The metering volume was the volume trapped between the valve seats when the two valve members were seated on their respective valve seats. A chamber around one valve was coupled to the gas reservoir, and the other valve opened into the gas laser tube or another tube coupled thereto. In operation, the "reservoir" valve was opened for a time sufficient to cause the metering volume to fill up with replacement gas at the pressure of the reservoir. After the metering volume was full, the "reservoir" valve was closed and the second or "laser tube" valve was opened thereby allowing the gas in the metering volume to be drawn into the laser tube, which has an internal pressure much lower than atmospheric pressure, to replenish the supply of gas therein.
The difficulty with this design was that two valves and corresponding driving mechanisms were necessary. This made the valve relatively expensive, and more parts were present to fail.
In an effort to simplify this design, workers in the art eliminated one of the valves and replaced it with a capillary tube having an inside diameter of five thousandths of an inch. This capillary tube had a diffusion constant which was longer than the interval during which the first valve coupled to the reservoir was opened. The capillary tube was connected to the interior of the laser gas tube. In operation, the first valve coupled to the reservoir was opened for a very short time which was shorter than the diffusion constant of the capillary tube. During this time the replacement gas from the reservoir filled the metering volume. The diffusion constant of the capillary tube had to be long compared to the time of opening of the first valve so that the capillary tube not appear as a leak. When this was true, the metering volume appeared to have no leak therein during the time that the valve was open, and the amount of gas that entered the metering volume could be accurately predicted. After the valve was closed and the diffusion time constant had passed, the gas trapped in the metering volume leaked into the laser tube through the capillary tube to replenish the laser tube ga supply.
One difficulty with this approach was that the capillary tube was difficult to keep clean. In order to have a diffusion constant which was smaller than the open time of the valve, it was necessary to use a capillary tube with a very small inside diameter. This made it extremely difficult to keep the bore of the capillary tube clean and resulted in contaminants in the capillary tube being sucked into the laser tube. The resultant contamination caused failure of laser tubes. Another difficulty with the capillary tube design was that the capillary tube acted as a virtual leak in the laser tube during the evacuation step of the process of manufacturing the laser tube. During manufacture of a gas laser, the tube is pumped down to the desired vacuum level prior to filling it with the desired gas. The virtual leak represented by the capillary caused the time to pump the laser tube down to the necessary level to be longer than would otherwise be necessary. Those skilled in the art will appreciate that the capillary tube coupled to the metering volume appears to be a crevice in the wall of the tube which couples into a cavern. In order for the laser to be effectively evacuated, all the gas molecules in the metering volume and the capillary tube had to be pumped out through the restrictive passageway presented by the capillary tube.
Thus a need exists for a simple, reliable, relatively inexpensive gas resupply valve for gas lasers.